1. Field of the Invention
This invention relates to sonar devices used on a boat to indicate the depth of the water. More particularly, this invention pertains to a solid state sonar apparatus for providing a digital read-out of bottom depth including means of more accurately controlling the gain of the receiver portion of the apparatus.
2. Description of the Prior Art
A commonly employed means of measuring and indicating the depth of water for use on a boat is a sonar apparatus. Sonar devices function by generating sequential bursts of sound energy, receiving energy reflected from the bottom surface, determining the depth as a function of the time which elapses from the transmission of sound energy until its reception as an echo, and displaying the depth determined on an indicating instrument.
In the past a method frequently employed for indicating depth utilizes a rotating disc having a gas filled tube carried by the disc which is illuminated when a bottom echo signals is received. A pulse of sound is transmitted at the beginning of each revolution of the disc and thereby the depth of the body of water in which the instrument is used is indicated by the angular displacement of the bulb at the time it is ignited by the received echo. Other devices for indicating depth include the use of a meter in which the depth is indicated by the quantity of current flow through the meter which in turn is proportional to the time duration required for the receipt of repeated transmitted and received sound signals.
A third type of depth indicator is a chart read-out in which a moving scriber passing over a record is energized to mark on the record in response to received echo signals. The markings on the record can be compared with depth indications on the chart as a means of indicating the bottom depth.
A fourth type of instrument for indicating the depth of water, and the type to which the present invention applies, is a digital read-out type in which the depth is indicated numerically by light emitting diode, liquid crystal, or other such devices. For certain applications, this type of depth indicator has several advantages over other types. First, by provision of a digital depth read-out, information is more precisely imparted to the viewer since it is a direct indication rather than an indirect wherein the reader must compare an indication to a scale. In addition, such digital type depth indicators may be constructed with no moving parts to thereby improve reliability and service life.
All types of sonar devices for indicating the depth of water in present use include some form of gain control. In the most rudimentary devices, gain control is manual so that the operator increases the gain as the anticipated bottom depth increases. To make the sonar devices more reliable and to remove the minute attention required by manual volume control, automatic gain control circuits have been devised and are in common use. As examples of such automatic gain control, reference may be had to U.S. Pat. Nos. 2,728,900 and 3,683,324. These types of gain controls have normally employed one form or another of time variance. In the typical sonar device having automatic gain control the gain is set at a minimum at the time of and immediately succeeding the transmission of a burst of sonic energy by the transducer. As time increases as the transducer awaits the receipt of an echo signal from the bottom, the receiver gain is increased on the basis of the fact that the longer the time required for the reflection of a signal, the weaker the signal will be when it is received. Such types of gain controls greatly improve the performance of sonar devices with which they are used. Most presently used systems of gain controls employ time varying gain superimposed over a manual control gain. However, there is no provision in such circuits for adjusting the gain control by taking into consideration a variety of other factors.
Some digital sonars have used automatic gain controls which set receiver gain to just pick up the reflected bottom signal. The principle is that it will be the strongest reflected signal, and thus all other weaker reflected signals will not be detected. However, the bottom signal is not always the strongest signal, and thus this system is not always reliable.
Digital sonar, as commonly employed, has not given consistent reliable performance because of the difficulty in discriminating in the circuitry a valid bottom return echo in contrast to reflections from fish, micro-organisms bubbles, thermoclines, etc.
It is an object of the present invention to provide an improved digital sonar having improved gain control characteristics.